(1) Field of the Invention
The present invention relates to an element and gate device which use a ferroelectric substance, especially to a ferroelectric element having improved dielectric polarization retention and squareness ratio, and a ferroelectric gate device using the same.
(2) Description of the Related Art
The recent developments in electronic devices are driving the need for increasingly larger data capacity. Nonvolatile memories are attracting attention to maintain data after power is turned off. There are several types of nonvolatile memories: flash memory, ferroelectric random-access memory (FeRAM), etc. When handling large amount of data at a high speed, a device may need a non-volatile memory which is still faster than these existing memories. For this reason, ferroelectric gate devices of Metal Ferroelectric Metal Insulator Semiconductor (MFMIS) type are attracting attention recently. An MFMIS type ferroelectric gate device has the problem of the distribution ratio of the voltage applied to a ferroelectric capacitor (ferroelectric thin film) and gate oxide. This problem will be explained with referring to FIG. 10.
FIG. 10(a) is a diagram of a circuit in which a paraelectric capacitor 101 is serially connected to a ferroelectric capacitor 102. The paraelectric capacitor 101 and ferroelectric capacitor 102 shown in FIG. 10(a) represent a gate oxide and a ferroelectric thin film in a ferroelectric gate device, respectively. One terminal of the ferroelectric capacitor 102 is grounded. Now, the voltage Vpp is applied to the terminal IN of the paraelectric capacitor 101. At this time, assume that the voltage at both ends of the ferroelectric capacitor 102 is Vf; the voltage at both ends of the paraelectric capacitor 101 is Vc; and the electric charge induced in each of the paraelectric capacitor 101 and the ferroelectric capacitor 102 is Q. The electric charge Q and voltage Vf of the ferroelectric capacitor 102 indicate the hysteresis characteristic as shown in FIG. 10(b). The relationship between the electric charge Q and voltage Vc of the paraelectric capacitor 101 is expressed as Formula 1.Q=CcVc=Cc(Vpp−Vf)  (Formula 1)The point A (see FIG. 10(b)), which is the intersection of the straight line expressed by Formula 1 and the above-mentioned hysteresis curve, is the operating point at this time.
When the voltage of the terminal IN, to which the voltage Vpp of the paraelectric capacitor 101 is applied, is returned to 0 V, the relationship between the electric charge Q and voltage Vc of the paraelectric capacitor 101 is expressed as Formula 2.Q=CcVc=−Ccvf  (Formula 2)The point B (see FIG. 10(b)), which is the intersection of the straight line expressed by Formula 2 and the above-mentioned hysteresis curve, is the operating point at this time. Since the polarization of the ferroelectric substance of the ferroelectric capacitor 102 is retained, the potential of −Vh is retained at the connection node of the paraelectric capacitor 101 and the ferroelectric capacitor 102.
To increase the retained voltage (−Vh), it is desirable to increase the voltage applied to the ferroelectric capacitor 102. However, when a voltage is applied to the terminal IN, a voltage will be also applied to the paraelectric capacitor 101. Therefore, the degree of the polarization of the ferroelectric substance does not become high enough. If the voltage applied to Terminal IN is too high, the electric field strength of the paraelectric capacitor 101 exceeds the withstand voltage. The coercive voltage can be increased by increasing the squareness ratio M of the ferroelectric substance (=Pr (remanence)/Ps (spontaneous polarization)) (see FIG. 10(b)). To do so, the crystallinity of the ferroelectric thin film must be improved. However, it is difficult to form a crystal of the ferroelectric thin film having the squareness ratio M as high as a bulk crystal of the ferroelectric substance.
As mentioned above, in the circuit where the paraelectric capacitor 101 and ferroelectric capacitor 102 are serially connected, there is a problem that the voltage retained at the connection node of both the capacitors 101 and 102 cannot be increased because it is difficult to apply a sufficiently high voltage only to a ferroelectric capacitor 102, and the squareness ratio of the ferroelectric thin film is not very large.